For the production of hydrocarbon wellbores are drilled into subterranean formations. Subsurface formations encountered in oil and gas drilling are compacted under in situ stresses due to overburden weight, tectonic effects, confinement and pore pressure. When the wellbore is drilled in a formation, the rock near the wellbore is subjected to increased shear stresses due to a reduction in confinement at the wellbore face after removal of the rock from the hole. Compressive failure of the rock near the wellbore will occur if the rock does not have sufficient strength to support the increased shear stresses imposed upon it.
Formation stability problems are not only encountered during the drilling of the wellbore. For the production of hydrocarbons, the hydrocarbon bearing formation is usually perforated or fractured to enable and stimulate the fluid flow into the wellbore. When producing from unconsolidated or weakly-consolidated reservoirs, the formation tends to produce particulates (e.g. sand) along with the hydrocarbons.
Formation sand is produced when the combined effects of fluid drag and near-wellbore stresses cause disaggregation near the perforation or fracture. Individual grains of sand are detached from the matrix forming the formation. At relatively low flow rates, fluid drag does not affect the stability, but as flow rate increases, drag forces become sufficiently high to remove sand particles from the matrix.
Flowrate from a formation is normally controlled by the perforation drawdown pressure (DP) which is the difference between the pore pressure (pw) in the formation and the bottomhole pressure (P0) and can hence be expressed as DP=P0−pw.
The critical drawdown pressure (CDP) is the value of DP at which the rock matrix surrounding the perforation begins to de-stabilize. Its value is determined by the maximum calculated rock strength.
To model the maximum rock strength classical elastic and elasto-plastic theories, failure criteria and fracture mechanics have been applied. Models use empirically or semi-empirically derived rock strength values to predict formation behavior by using classical theories and stress, pore pressure and empirically derived strength data from various wells.
There are several methods for predicting when for example sand production will occur in a particular well. Such methods are disclosed and discussed in the U.S. Pat. No. 5,497,658 and references contained therein. Known rock failure criteria as discussed in this and other published document are referred to as Mohr-Coulomb, critical state, Drucker-Pager model or as extended Von Mises criterion
To apply the failure criteria it is necessary to measure rock properties and the formation fluid properties from core samples, wellbore logs, and the like.
It is therefore an object of the present invention to provide a novel method of estimating the strength of cavities in the subterranean formation, particulary the initiation of sand production in subterranean (sandstone) formations.